University of Michigan scientists did their research on nine lab rats that were anesthetised and then subjected to induced cardiac arrest as part of the experiment.

In the first 30 seconds after their hearts were stopped, they all showed a surge of brain activity, observed in electroencephalograms (EEGs) that indicated highly aroused mental states.

"We were surprised by the high levels of activity," says senior author George Mashour, professor of anesthesiology and neurosurgery at the University of Michigan.

"In fact, at near-death, many known electrical signatures of consciousness exceeded levels found in the waking state, suggesting that the brain is capable of well-organized electrical activity during the early stage of clinical death."

Similar results in terms of brain activity were seen in rats that were asphyxiated, the researchers write.

"This study tells us that reduction of oxygen or both oxygen and glucose during cardiac arrest can stimulate brain activity that is characteristic of conscious processing," says lead author Jimo Borjigin.

"It also provides the first scientific framework for the near-death experiences reported by many cardiac arrest survivors."

About 20 per cent of people who survive cardiac arrest report having had visions during a period known to doctors as clinical death.

Borjigin says she hopes her team's latest study "will form the foundation for future human studies investigating mental experiences occurring in the dying brain, including seeing light during cardiac arrest."

Animal consciousness

Mainstream science has long considered the brain to be inactive during this period, and some experts questioned how much a study on rats can truly reveal about the human brain.

"Do we know if animals experience 'consciousness'? Most philosophers and scientists are still at loggerheads over what the term refers to in humans, let alone in other species," says David McGonigle, a lecturer at Cardiff University.

"While recent research now suggests that animals may indeed have the kind of autobiographical memories that humans possess - the kinds of memories that allow us to place ourselves in a certain time and place - it seems unlikely that near death experiences would necessarily be similar across species."

Anders Sandberg, a research fellow at Oxford University described the research as "simple" and "well-done", but urged caution in interpreting the results.

"EEG tells us things about brain activity a bit like listening at traffic noise tells you what is going on in a city. It is certainly informative, but also an average of a lot of individual interactions," he says.

"No doubt some people will presumptuously claim that this is further evidence for life after death, which is doubly silly. Near-death experiences are in themselves just experiences."

"But if one believes that, then one should also conclude the afterlife includes a lot of lab mice."